I would use an easy out, tap it into whats left of the allen head. The use a small adjustable wrench on the end of the easy out (they are usually squared off) and unscrew the bolt. May take a couple of tries.

Final solution is above, grind it off and lift the head over the remain shank. Then use a pair of vice grips and crank it out.

Got to work at it.

Good luck.

Cheers,

Chip